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  <title>源码分析 | ClickHouse和他的朋友们 (８) 纯手工打造的SQL解析器 | DBKernel - 专注于分享数据库技术</title>
  <meta name="description" content="本文首发于 2020-07-26 21:55:10  《ClickHouse和他的朋友们》系列文章转载自圈内好友 BohuTANG 的博客，原文链接：https:&#x2F;&#x2F;bohutang.me&#x2F;2020&#x2F;07&#x2F;25&#x2F;clickhouse-and-friends-parser&#x2F;以下为正文。  现实生活中的物品一旦被标记为“纯手工打造”，给人的第一感觉就是“上乘之品”，一个字“贵”，比如北京老布鞋。">
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<meta property="og:description" content="本文首发于 2020-07-26 21:55:10  《ClickHouse和他的朋友们》系列文章转载自圈内好友 BohuTANG 的博客，原文链接：https:&#x2F;&#x2F;bohutang.me&#x2F;2020&#x2F;07&#x2F;25&#x2F;clickhouse-and-friends-parser&#x2F;以下为正文。  现实生活中的物品一旦被标记为“纯手工打造”，给人的第一感觉就是“上乘之品”，一个字“贵”，比如北京老布鞋。">
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      源码分析 | ClickHouse和他的朋友们 (８) 纯手工打造的SQL解析器
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<p><strong>本文首发于 2020-07-26 21:55:10</strong></p>
<blockquote>
<p>《ClickHouse和他的朋友们》系列文章转载自圈内好友 <a target="_blank" rel="noopener" href="https://bohutang.me/">BohuTANG</a> 的博客，原文链接：<br><a target="_blank" rel="noopener" href="https://bohutang.me/2020/07/25/clickhouse-and-friends-parser/">https://bohutang.me/2020/07/25/clickhouse-and-friends-parser/</a><br>以下为正文。</p>
</blockquote>
<p>现实生活中的物品一旦被标记为“纯手工打造”，给人的第一感觉就是“上乘之品”，一个字“贵”，比如北京老布鞋。</p>
<p>但是在计算机世界里，如果有人告诉你 ClickHouse 的 SQL 解析器是纯手工打造的，是不是很惊讶！</p>
<p>这个问题引起了不少网友的关注，所以本篇聊聊 ClickHouse 的纯手工解析器，看看它们的底层工作机制及优缺点。</p>
<p>枯燥先从一个 SQL 开始：</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">EXPLAIN <span class="keyword">SELECT</span> a,b <span class="keyword">FROM</span> t1</span><br></pre></td></tr></table></figure>

<h2 id="token"><a href="#token" class="headerlink" title="token"></a>token</h2><p>首先对 SQL 里的字符逐个做判断，然后根据其关联性做 token 分割：</p>
<p><img src="parser.png" alt="parser.png"></p>
<p>比如连续的 WordChar，那它就是 BareWord，解析函数在 <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/Lexer.cpp#L61">Lexer::nextTokenImpl()</a>，解析调用栈：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">DB::Lexer::<span class="built_in">nextTokenImpl</span>() Lexer.cpp:<span class="number">63</span></span><br><span class="line">DB::Lexer::<span class="built_in">nextToken</span>() Lexer.cpp:<span class="number">52</span></span><br><span class="line">DB::Tokens::<span class="keyword">operator</span>[](<span class="keyword">unsigned</span> <span class="keyword">long</span>) TokenIterator.h:<span class="number">36</span></span><br><span class="line">DB::TokenIterator::<span class="built_in">get</span>() TokenIterator.h:<span class="number">62</span></span><br><span class="line">DB::TokenIterator::<span class="keyword">operator</span>-&gt;() TokenIterator.h:<span class="number">64</span></span><br><span class="line">DB::<span class="built_in">tryParseQuery</span>(DB::IParser&amp;, <span class="keyword">char</span> <span class="keyword">const</span>*&amp;, <span class="keyword">char</span> <span class="keyword">const</span>*, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt;&amp;, <span class="keyword">bool</span>, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, <span class="keyword">bool</span>, <span class="keyword">unsigned</span> <span class="keyword">long</span>, <span class="keyword">unsigned</span> <span class="keyword">long</span>) parseQuery.cpp:<span class="number">224</span></span><br><span class="line">DB::<span class="built_in">parseQueryAndMovePosition</span>(DB::IParser&amp;, <span class="keyword">char</span> <span class="keyword">const</span>*&amp;, <span class="keyword">char</span> <span class="keyword">const</span>*, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, <span class="keyword">bool</span>, <span class="keyword">unsigned</span> <span class="keyword">long</span>, <span class="keyword">unsigned</span> <span class="keyword">long</span>) parseQuery.cpp:<span class="number">314</span></span><br><span class="line">DB::<span class="built_in">parseQuery</span>(DB::IParser&amp;, <span class="keyword">char</span> <span class="keyword">const</span>*, <span class="keyword">char</span> <span class="keyword">const</span>*, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, <span class="keyword">unsigned</span> <span class="keyword">long</span>, <span class="keyword">unsigned</span> <span class="keyword">long</span>) parseQuery.cpp:<span class="number">332</span></span><br><span class="line">DB::<span class="built_in">executeQueryImpl</span>(<span class="keyword">const</span> <span class="keyword">char</span> *, <span class="keyword">const</span> <span class="keyword">char</span> *, DB::Context &amp;, <span class="keyword">bool</span>, DB::QueryProcessingStage::Enum, <span class="keyword">bool</span>, DB::ReadBuffer *) executeQuery.cpp:<span class="number">272</span></span><br><span class="line">DB::<span class="built_in">executeQuery</span>(DB::ReadBuffer&amp;, DB::WriteBuffer&amp;, <span class="keyword">bool</span>, DB::Context&amp;, std::__1::function&lt;<span class="built_in"><span class="keyword">void</span></span> (std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;, std::__1::basic_string&lt;<span class="keyword">char</span>, std::__1::char_traits&lt;<span class="keyword">char</span>&gt;, std::__1::allocator&lt;<span class="keyword">char</span>&gt; &gt; <span class="keyword">const</span>&amp;)&gt;) executeQuery.cpp:<span class="number">731</span></span><br><span class="line">DB::MySQLHandler::<span class="built_in">comQuery</span>(DB::ReadBuffer&amp;) MySQLHandler.cpp:<span class="number">313</span></span><br><span class="line">DB::MySQLHandler::<span class="built_in">run</span>() MySQLHandler.cpp:<span class="number">150</span></span><br></pre></td></tr></table></figure>

<h2 id="ast"><a href="#ast" class="headerlink" title="ast"></a>ast</h2><p>token 是最基础的元组，他们之间没有任何关联，只是一堆生冷的词组与符号，所以我们还需对其进行<strong>语法解析</strong>，让这些 token 之间建立一定的关系，达到一个可描述的活力。</p>
<p>ClickHouse 在解每一个 token 的时候，会根据当前的 token 进行状态空间进行预判（parse 返回 true 则进入子状态空间继续），然后决定状态跳转，比如：</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">EXPLAIN  <span class="comment">-- TokenType::BareWord</span></span><br></pre></td></tr></table></figure>

<p>逻辑首先会进入Parsers/ParserQuery.cpp 的 <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserQuery.cpp#L26">ParserQuery::parseImpl</a> 方法：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> res = query_with_output_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || insert_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || use_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || set_role_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || set_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || system_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || create_user_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || create_role_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || create_quota_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || create_row_policy_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || create_settings_profile_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || drop_access_entity_p.<span class="built_in">parse</span>(pos, node, expected)</span><br><span class="line">    || grant_p.<span class="built_in">parse</span>(pos, node, expected);</span><br></pre></td></tr></table></figure>

<p>这里会对所有 query 类型进行 parse 方法的调用，直到有分支返回 true。</p>
<p>我们来看<strong>第一层</strong> query_with_output_p.parse <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserQueryWithOutput.cpp#L31">Parsers/ParserQueryWithOutput.cpp</a>：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> parsed =</span><br><span class="line">       explain_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || select_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_create_access_entity_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_tables_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || table_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || describe_table_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_processlist_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || create_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || alter_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || rename_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || drop_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || check_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || kill_query_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || optimize_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || watch_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_access_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_access_entities_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_grants_p.<span class="built_in">parse</span>(pos, query, expected)</span><br><span class="line">    || show_privileges_p.<span class="built_in">parse</span>(pos, query, expected</span><br></pre></td></tr></table></figure>

<p>跳进<strong>第二层</strong> explain_p.parse <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserExplainQuery.cpp#L10">ParserExplainQuery::parseImpl</a>状态空间：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">bool</span> <span class="title">ParserExplainQuery::parseImpl</span><span class="params">(Pos &amp; pos, ASTPtr &amp; node, Expected &amp; expected)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    ASTExplainQuery::ExplainKind kind;</span><br><span class="line">    <span class="keyword">bool</span> old_syntax = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_ast</span><span class="params">(<span class="string">&quot;AST&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_analyze</span><span class="params">(<span class="string">&quot;ANALYZE&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_explain</span><span class="params">(<span class="string">&quot;EXPLAIN&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_syntax</span><span class="params">(<span class="string">&quot;SYNTAX&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_pipeline</span><span class="params">(<span class="string">&quot;PIPELINE&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_plan</span><span class="params">(<span class="string">&quot;PLAN&quot;</span>)</span></span>;</span><br><span class="line"></span><br><span class="line">    ... ...</span><br><span class="line">    <span class="keyword">else</span> <span class="keyword">if</span> (s_explain.<span class="built_in">ignore</span>(pos, expected))</span><br><span class="line">    &#123;</span><br><span class="line">       ... ...</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    ... ...</span><br><span class="line"></span><br><span class="line">    ParserSelectWithUnionQuery select_p;</span><br><span class="line">    ASTPtr query;</span><br><span class="line">    <span class="keyword">if</span> (!select_p.<span class="built_in">parse</span>(pos, query, expected))</span><br><span class="line">        <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line">    ... ...</span><br></pre></td></tr></table></figure>

<p>s_explain.ignore 方法会进行一个 keyword 解析，解析出 ast node:</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">EXPLAIN <span class="comment">-- keyword</span></span><br></pre></td></tr></table></figure>

<p>跃进<strong>第三层</strong> select_p.parse <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserSelectWithUnionQuery.cpp#L26">ParserSelectWithUnionQuery::parseImpl</a>状态空间：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">bool</span> <span class="title">ParserSelectWithUnionQuery::parseImpl</span><span class="params">(Pos &amp; pos, ASTPtr &amp; node, Expected &amp; expected)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    ASTPtr list_node;</span><br><span class="line"></span><br><span class="line">    <span class="function">ParserList <span class="title">parser</span><span class="params">(std::make_unique&lt;ParserUnionQueryElement&gt;(), std::make_unique&lt;ParserKeyword&gt;(<span class="string">&quot;UNION ALL&quot;</span>), <span class="literal">false</span>)</span></span>;</span><br><span class="line">    <span class="keyword">if</span> (!parser.<span class="built_in">parse</span>(pos, list_node, expected))</span><br><span class="line">        <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line">...</span><br></pre></td></tr></table></figure>

<p>parser.parse 里又调用<strong>第四层</strong> <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserSelectQuery.cpp#L24">ParserSelectQuery::parseImpl</a> 状态空间：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">bool</span> <span class="title">ParserSelectQuery::parseImpl</span><span class="params">(Pos &amp; pos, ASTPtr &amp; node, Expected &amp; expected)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">auto</span> select_query = std::make_shared&lt;ASTSelectQuery&gt;();</span><br><span class="line">    node = select_query;</span><br><span class="line"></span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_select</span><span class="params">(<span class="string">&quot;SELECT&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_distinct</span><span class="params">(<span class="string">&quot;DISTINCT&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_from</span><span class="params">(<span class="string">&quot;FROM&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_prewhere</span><span class="params">(<span class="string">&quot;PREWHERE&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_where</span><span class="params">(<span class="string">&quot;WHERE&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_group_by</span><span class="params">(<span class="string">&quot;GROUP BY&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_with</span><span class="params">(<span class="string">&quot;WITH&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_totals</span><span class="params">(<span class="string">&quot;TOTALS&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_having</span><span class="params">(<span class="string">&quot;HAVING&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_order_by</span><span class="params">(<span class="string">&quot;ORDER BY&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_limit</span><span class="params">(<span class="string">&quot;LIMIT&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_settings</span><span class="params">(<span class="string">&quot;SETTINGS&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_by</span><span class="params">(<span class="string">&quot;BY&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_rollup</span><span class="params">(<span class="string">&quot;ROLLUP&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_cube</span><span class="params">(<span class="string">&quot;CUBE&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_top</span><span class="params">(<span class="string">&quot;TOP&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_with_ties</span><span class="params">(<span class="string">&quot;WITH TIES&quot;</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserKeyword <span class="title">s_offset</span><span class="params">(<span class="string">&quot;OFFSET&quot;</span>)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ParserNotEmptyExpressionList <span class="title">exp_list</span><span class="params">(<span class="literal">false</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserNotEmptyExpressionList <span class="title">exp_list_for_with_clause</span><span class="params">(<span class="literal">false</span>)</span></span>;</span><br><span class="line">    <span class="function">ParserNotEmptyExpressionList <span class="title">exp_list_for_select_clause</span><span class="params">(<span class="literal">true</span>)</span></span>;</span><br><span class="line">    ...</span><br><span class="line"></span><br><span class="line">            <span class="keyword">if</span> (!exp_list_for_select_clause.<span class="built_in">parse</span>(pos, select_expression_list, expected))</span><br><span class="line">            <span class="keyword">return</span> <span class="literal">false</span>;</span><br></pre></td></tr></table></figure>

<p><strong>第五层</strong> exp_list_for_select_clause.parse <a target="_blank" rel="noopener" href="https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ExpressionListParsers.cpp#L520">ParserExpressionList::parseImpl</a>状态空间继续：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">bool</span> <span class="title">ParserExpressionList::parseImpl</span><span class="params">(Pos &amp; pos, ASTPtr &amp; node, Expected &amp; expected)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="built_in">ParserList</span>(</span><br><span class="line">        std::make_unique&lt;ParserExpressionWithOptionalAlias&gt;(allow_alias_without_as_keyword),</span><br><span class="line">        std::make_unique&lt;ParserToken&gt;(TokenType::Comma))</span><br><span class="line">        .<span class="built_in">parse</span>(pos, node, expected);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>… … 写不下去个鸟！</p>
<p>可以发现，ast parser 的时候，预先构造好状态空间，比如 select 的状态空间:</p>
<ol>
<li>expression list</li>
<li>from tables</li>
<li>where</li>
<li>group by</li>
<li>with …</li>
<li>order by</li>
<li>limit</li>
</ol>
<p>在一个状态空间內，还可以根据 parse 返回的 bool 判断是否继续进入子状态空间，一直递归解析出整个 ast。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>手工 parser 的好处是代码清晰简洁，每个细节可防可控，以及友好的错误处理，改动起来不会一发动全身。</p>
<p>缺点是手工成本太高，需要大量的测试来保证其正确性，还需要一些fuzz来保证可靠性。</p>
<p>好在ClickHouse 已经实现的比较全面，即使有新的需求，在现有基础上修修补补即可。</p>
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<thead>
<tr>
<th>标题</th>
<th>网址</th>
</tr>
</thead>
<tbody><tr>
<td>GitHub</td>
<td><a href="https://dbkernel.github.io/">https://dbkernel.github.io</a></td>
</tr>
<tr>
<td>知乎</td>
<td><a target="_blank" rel="noopener" href="https://www.zhihu.com/people/dbkernel/posts">https://www.zhihu.com/people/dbkernel/posts</a></td>
</tr>
<tr>
<td>思否（SegmentFault）</td>
<td><a target="_blank" rel="noopener" href="https://segmentfault.com/u/dbkernel">https://segmentfault.com/u/dbkernel</a></td>
</tr>
<tr>
<td>掘金</td>
<td><a target="_blank" rel="noopener" href="https://juejin.im/user/5e9d3ed251882538083fed1f/posts">https://juejin.im/user/5e9d3ed251882538083fed1f/posts</a></td>
</tr>
<tr>
<td>开源中国（oschina）</td>
<td><a target="_blank" rel="noopener" href="https://my.oschina.net/dbkernel">https://my.oschina.net/dbkernel</a></td>
</tr>
<tr>
<td>博客园（cnblogs）</td>
<td><a target="_blank" rel="noopener" href="https://www.cnblogs.com/dbkernel">https://www.cnblogs.com/dbkernel</a></td>
</tr>
</tbody></table>

      
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